1. Field of the Invention
The present invention relates to a solid-state imaging device. More particularly, the present invention relates to an imaging apparatus to be used for a digital still camera, a digital video camera or the like.
2. Description of the Related Art
A solid-state imaging device formed by arranging ranging pixels having a ranging function as part or all of the pixels of the imaging device so as to detect the state of focus adjustment by way of a phase difference detection type process has been proposed (see Japanese Patent No. 4,797,606). A ranging pixel has a microlens and a light shielding plate arranged at a position that is optically conjugated with the exit pupil of the camera lens by way of the microlens. With this arrangement, a light flux that passes through a partial area on the pupil of the camera lens is guided to a photoelectric conversion section. Additionally, ranging pixels are provided to receive light fluxes in areas that are decentered in opposite directions relative to each other from the center of the pupil of the camera lens, which serves as reference position. Then, a plurality of optical images (to be referred to as ranging images hereinafter) generated by the light fluxes that pass through different pupil areas can be acquired by means of the signals obtained by the ranging pixels. Thus, the distance to an object can be measured by computing the quantity of defocus by means of triangulation using 3D images on the basis of the quantities of displacement of the acquired ranging images. With such a proposed device, the distance to the object is measured unlike conventional contrast detection type devices and hence the lens of the camera including such a device does not need to be moved for focusing. In other words, high speed and high accuracy ranging operations can be realized with such a device.
However, a device as described above is accompanied by a problem that the image quality of the obtained ranging images is poor to consequently degrade the ranging accuracy depending on the shooting conditions of the camera including such a device. Generally, the exit pupil of a camera lens changes its position according to its conditions in terms of zooming and focusing. Therefore, when the light shielding plate of a ranging pixel gets out of the conjugate relationship with the exit pupil, the quantity of decentering of the pupil area through which the light flux to be received by the ranging pixel passes varies as a function of the position of the ranging pixel in the imaging device. As the quantity of decentering increases, a ranging image is formed only by a light flux that passes through a peripheral section in the pupil of the camera lens. Generally, since light fluxes that pass through peripheral sections in the pupil of a camera lens are accompanied by large aberrations if compared with light fluxes that pass through center sections of the pupil, the ranging images formed by the former light fluxes are apt to represent a duller image contrast due to the aberrations. Using poor quality ranging images entails large reading errors in the quantities of image displacements to by turn reduce the ranging accuracy.